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Solutes and Solution Solutes and Solution The first rule of solubility is “likes dissolve likes” Polar or ionic substances are soluble in polar solvents Non-polar substances are soluble in non- polar solvents Solutes and Solution There must be a reason why a substance is soluble in a solvent: either the solution process lowers the overall enthalpy of the system (Hrxn < 0) Or the solution process increases the overall entropy of the system (Srxn > 0) Entropy is a measure of the amount of disorder in a system—entropy must increase for any spontaneous change 1 Solutes and Solution The forces that drive the dissolution of a solute usually involve both enthalpy and entropy terms Hsoln < 0 for most species The creation of a solution takes a more ordered system (solid phase or pure liquid phase) and makes more disordered system (solute molecules are more randomly distributed throughout the solution) Saturation and Equilibrium If we have enough solute available, a solution can become saturated—the point when no more solute may be accepted into the solvent Saturation indicates an equilibrium between the pure solute and solvent and the solution solute + solvent solution KC 2 Saturation and Equilibrium solute + solvent solution KC The magnitude of KC indicates how soluble a solute is in that particular solvent If KC is large, the solute is very soluble If KC is small, the solute is only slightly soluble Saturation and Equilibrium Examples: + - NaCl(s) + H2O(l) Na (aq) + Cl (aq) KC = 37.3 A saturated solution of NaCl has a [Na+] = 6.11 M and [Cl-] = 6.11 M + - AgCl(s) + H2O(l) Ag (aq) + Cl (aq) -10 KC = 1.8 x 10 A saturated solution of AgCl has a [Ag+] = 1.34 x 10-5 M and [Cl-] 1.34 x 10-5 M 3 Saturation and Equilibrium Saturation and Equilibrium 4 Solubility of Gases When a gas is dissolved in a solvent, the process is usually exothermic: gas + solvent solution Hrxn < 0 In solution, the gas molecules are now in close proximity to the solvent molecules which increases the effects of the intermolecular forces and lowers the overall enthalpy of the system Solubility of Gases To understand the effects of temperature on solubility, we can use LeChatlier’s Principle: gas + solvent solution + heat Since heat appears on the product side of the chemical equilibrium, increasing the temperature will move the equilibrium toward the reactants and the gas will be less soluble 5 Solubility of Gases Pressure has no effect on the solubility of solids and liquids, but it has a significant effect on the solubility of gases The higher the pressure of the gas over the solution, the higher the solubility of te gas in solution Henry’s Law: Sg = kH Pg Solubility of Gases Henry’s Law: Sg = kH Pg Sg = solubility of gas in solution kH = Henry’s Law constant for that gas/solvent combination Pg = partial pressure of gas over the solution 6 Solubility of Gases Example: What is the molar concentration of N2 in water when the partial pressure of N2 is 600. Torr? -7 kH = 8.4 x 10 mol/Lmm Hg -7 SN2 = (8.4 x 10 mol/Lmm Hg)(600. mm Hg) = 5.04 x 10-4 M What is the concentration at 20. Torr? -7 SN2 = (8.4 x 10 mol/Lmm Hg)(20. mm Hg) = 1.7 x 10-5 M Solubility of Gases 7 Concentration Units Molarity: defined as moles of solute per liter of solution moles solute mol M = = liters of solution L The volume used is the total volume of the solution, not just the volume of the solvent Usually the volume of the solute is negligible compared to the solvent volume Concentration Units Molality: defined as moles of solute per mass of solvent moles solute mol m = = kilograms of solvent kg In this case, the mass is only the mass of the solvent, not the total mass of the solution 8 Concentration Units Mole fraction: mole fraction is defined as the number of moles solute divided by the total number of moles of all species in the solution moles solute nA X A = = total moles in solution ntot Mole fraction is a unitless number Colligative Properties Colligative properties are a set properties that depend only on the amount of solute in a solution, and not on the chemical identity of the solute Colligative properties include: Vapor pressure lowering Freezing point depression Boiling point elevation Osmotic pressure 9 Colligative Properties Vapor pressure lowering When solute is added to a pure solvent, solvent molecules are “tied up” in keeping the solute molecules in solution Because solvent molecules are more strongly attracted to the solute than to themselves, it requires more energy to remove them from the solution compared to the pure solvent Colligative Properties Vapor pressure lowering As a consequence, the vapor pressure of the solution is lowered Raoult’s Law states: o P1 = X1 P1 P1 = vapor pressure of the solution X1 = mole fraction of solvent o P1 = vapor pressure of the pure solvent 10 Colligative Properties Vapor pressure lowering Example: What is the vapor pressure of a saturated NaCl solution at 25 oC? Po = 23.76 Torr H2O = 0.99707 g/mL 35.7 g NaCl per 100 mL H2O Step 1—Determine mole fraction of solution 35.7 g NaCl/58.443 g/mol =0.611 mol NaCl (100 mL)(.9971 g/mL)/(18.0152 g.mol) =5.53 mol H2O Colligative Properties Vapor pressure lowering Example: What is the vapor pressure of a saturated NaCl solution at 25 oC? Step 1—Determine mole fraction of solution 5.53 mol X = = .819 2(.611 mol) + 5.53 mol 11 Colligative Properties Vapor pressure lowering Example: What is the vapor pressure of a saturated NaCl solution at 25 oC? Step 2—Determine vapor pressure of solution P = X Po = (.819)(23.76 Torr) = 19.5 Torr The vapor pressure over a saturated NaCl solution is nearly 20% lower than that of pure water Colligative Properties Boiling Point Elevation Because the vapor pressure of solution is lower than the vapor pressure of the pure solvent, the solution’s boiling point will be elevated Remember that the boiling point is that temperature where the vapor pressure of the solution is equal to the pressure over the solution 12 Colligative Properties Boiling Point Elevation Tb = Tb(soln) – Tb(solvent) = Kb m Kb = boiling point elevation constant m = molality of solute Colligative Properties Boiling Point Elevation Example: What is boiling point of a NaCl solution that is saturated at 25 oC? Kb(H2O) = 0.512 K kg/mol m = (1.22 mol solute)/(.09771 kg H2O) = 12.5 m Tb = (12.5 mol/kg)(0.512 K kg/mol) = 6.40 K o Tb = 106.4 C 13 Colligative Properties Freezing Point Depression The freezing point of a solution will be lower than that of the pure solvent because the solute molecules interrupt the crystal structure of the solid solvent Tf = Kf m Kf = freezing pt depression constant m = molality of solute Colligative Properties Freezing Point Depression Example: Determine the freezing point of a solution that is 40% by volume ethylene glycol in water Kf(H2O) = 1.86 K kg/mol (C2H6O2) =1.109 g/mL 14 Colligative Properties Freezing Point Depression Example: Determine the freezing point of a solution that is 40% by volume ethylene glycol in water Step 1—Assume we have 1.00 L of the solution; determine molality of ethylene glycol (400 mL C2H6O2)(1.109 g/mL)/(62.069 g/mol) = 7.15 mol C2H6O2 m = (7.15 mol)/(.600 kg H2O) =11.9 m Colligative Properties Freezing Point Depression Example: Determine the freezing point of a solution that is 40% by volume ethylene glycol in water Step 2—Determine freezing point depression Tf = (11.9 m)(1.86 K kg/mol) = 22.1 K o Tf = -22.1 C 15 Colligative Properties Osmotic Pressure A solution is initially separated from a reservoir of pure solvent by a semi- pure solution permeable membrane solvent A semi-permeable membrane allows the solvent to move through in either direction, but does not allow the solute species to pass through semi-permeable membrane Colligative Properties Osmotic Pressure The solvent wants to have equal concentrations on solution each side of the system The solvent will flow from pure the pure solvent side to the solvent solution side of the system to minimize the flow of solvent concentration difference between each side of the system semi-permeable membrane 16 Colligative Properties Osmotic Pressure The solution side is now higher than the solvent side solution which creates a pressure difference pure solvent The increased pressure on the solution side tends to push solvent back to the pure solvent side of the system semi-permeable membrane Colligative Properties Osmotic Pressure The osmotic pressure of the solution is the pressure solution exerted by the solution when the system reaches pure equilibrium—balance solvent between force exerted by pure solvent to equalize concentration and force exerted by increased height of solution semi-permeable membrane 17 Colligative Properties Osmotic Pressure The osmotic pressure is given by: solution = c R T i pure where solvent c = molar concentration R = gas constant T = temperature (in K) i = solute particles per unit formula semi-permeable membrane Colligative Properties Osmotic Pressure Example: Determine the osmotic pressure of a 1.00 M solution of NaCl c = 1.00 mol/L i = 2 (Na+ and Cl-) = 2(1.00 mol/L)(.08206 L atm/mol K)(298 K) = 48.9 atm 18.
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